The present invention relates to a method of transmitting dual tone multi-frequency (hereinafter, called DTMF) during communication with a central office line (hereinafter, referred to as "C.O. line") using a keyphone in a private branch exchange (PBX), specifically of sending DTMF to the C.O. line on `busy` state by controling DTMF sender in the PBX when the dialing digit keys of a keyphone are pressed.
At present, conventional PBXs have interface circuits that allow interface between the PBX and subscribers of ordinary-telephone extensions and subscribers of digital keyphones that can send digital data and voice signal, or of ordinary keyphones. According to the conventional method said digital keyphones or said ordinary keyphones of subscribers have two methods of sending dialing signals, that is, the one that sends dialing signal by having a built-in DTMF sender that can generate DTMF therein, and the other in which a built-in micro-processor recognizes the key-data of the key matrix that can generate dial digit and other functional data, changes the dialing key-signals to digital data and send it to the PBX. The former method, however, needs built-in DTMF sender in every telephone of subscribers, therefore it not only adds a cost rise factor but also causes some problems in sending dialing signals. The reason is that all the signals must be transformed into digital signals to be sent because all the signal transmissions are processed in the form of the digital signals in digital keyphones.
Therefore, in the case of keyphones, the latter method is gradually becoming popular that a built-in micro-processor sends data, such as dial key signals and digital signals of the key matrix from which function key signals can be generated at a key-press, in the form of dial data. But there has been still a problem described below in such a method that performs telephonic communication by entering dial digit data signals into PBX. When a subscriber has to send DTMF to the C.O. line to operate an answering phone or a home automation machine at the time of making a call towards the C.O. line from a keyphone in PBX, he has to face the problem of being unable to control those devices which operate by receiving DTMF, for there is no DTMF generator in the keyphone itself being used.
Therefore, a purpose of this invention is to provide keyphone users in a PBX system with a DTMF transmission method that allows them to send DTMF of the dial digits that they pressed, through a DTMF sender built in the PBX itself, even though there is no DTMF generator in the keyphone itself being used at present, when the keyphone user needs to send DTMF to the C.O. line to control the devices that operate at the reception of DTMF during making a call to the C.O. line.